1. Field of the Invention
This invention generally relates to instrument transformers and more particular to instrument transformers having a plurality of primary or secondary windings exteriorly terminated on different faces of the instrument transformer housing.
2. Description of the Prior Art
Instrument transformers are generally known in the art and are typically used in metering and control circuits. They function to convert primary voltages and currents to values which are suitable for use in various types of meters, relays, control circuits and the like. Instrument transformers have been used in polyphase alternating current applications where the magnitude of the primary voltage or current is greater than 480 volts or 5 amperes, respectively. At lower magnitudes, the primary currents and voltages may be used directly. Alternatively, voltage divider networks and current shunts are used. However, use of such devices results in relatively large power dissipation and consequently such use is expensive.
Essentially, there are two types of instrument transformers: potential transformers and current transformers. Potential transformers have been used for measuring the voltage in a primary circuit and current transformers have been used for measuring the current in a primary circuit.
Both potential and current transformers are used in a wide variety of applications. For example, potential and current transformers have been used as pilot devices for voltmeters, ammeters, power factor meters, watt meters, demand meters, watt hour meters and the like. They have also been used in a variety of control circuits ranging from circuits for controlling motors to circuits for controlling the operation of metal clad switchgear breakers.
Although potential transformers and current transformers are similar in construction, they differ in their application. For example, potential transformers are connected across line conductors in a primary circuit or between a line conductor in a primary circuit and a ground conductor in a grounded system. The current flowing through the primary winding of the potential transformer is substantially less than the line current in the primary circuit. The primary winding of current transformers is normally connected in series with a line conductor. Thus, the primary winding of a current transformer will be exposed to the line current of the primary circuit.
Various conventional constructions of instrument transformers have been used. Typically, instrument potential transformers are of the wound primary type consisting of a wound primary winding and a wound secondary winding insulated from each other and permanently assembled on a laminated core. Current transformers are comprised of various types of constructions. One type is a wound primary type, similar to potential transformers. Another type is the bar type, similar to the wound primary type, except the primary winding is a single conductor. A window type, consisting only of a secondary winding, uses a line conductor as its primary winding.
Recently, there has been a trend to utilize electronic controls for molded case circuit breakers. In such electronically controlled circuit breakers, a current transformer is used to provide a current source compatible with the electronic control circuit. Due to the compactness of molded case circuit breaker, the current transformers have sometimes been mounted external to the molded case circuit breaker housing and hand wired to the electronic control circuit for the molded case circuit breaker. This results in additional labor required to install the current transformer and wire it to the electronic control circuit. Moreover, the current transformers used with such electronically controlled molded case circuit breakers are often provided with a plurality of transformation ratios which can only be changed by interconnecting portions of the windings by various means. Therefore, changing of the transformation ratios of a current transformer in the field has been quite cumbersome.
Thus, there exists a need to provide an instrument transformer having a plurality of transformation ratios which can be changed quickly and easily. There is also a need for a dimensionally small instrument transformer adapted to be received in an electronically controlled molded case circuit breaker housing for supplying current proportional to the primary current through the circuit breaker.